The present invention relates to microthermionic self-powered converters having high energy conversion efficiencies and to methods of manufacturing those converters using micromachining manufacturing techniques.
Thermionic generators were first proposed in 1915 by Schlichter, but many of the theoretical problems that existed at the inception of the idea persist today. Thermionic generators convert heat energy to electrical energy by an emission of electrons from a heated emitter electrode. The electrons flow from the emitter electrode, across an interelectrode gap, to a collector electrode, through an external load, and return back to the emitter electrode, thereby converting the heat energy to electrical energy. Historically, voltages produced are low, and the high temperature required to produce adequate current has produced numerous problems in maintaining the devices, including the unintended transfer of heat from the heated emitter electrode to the cold collector electrode. Practical thermionic conversion was demonstrated in 1957 by Hernquist in which efficiencies of 5-10% were reached with power densities of 3-10 W/cm2. Generally, such efficiencies and power densities were not sufficient to be financially competitive in the energy market, thus reducing the application of such devices. Furthermore, such devices were too large for use as miniaturized electrical power sources.
Another problem, xe2x80x9cspace-charge effect,xe2x80x9d is described by Edelson (U.S. Pat. No. 5,994,638). A space-charge effect results when the build up of negative charge in the cloud of electrons between the two electrodes deter the movement of other electrons toward the collector electrode. Edelson cites two well-known methods for mitigating the space-charge effect: (1) reducing the spacing between electrodes to the order of microns, or (2) introducing positive ions into the electron cloud in front of the emitter electrode.
Introducing positive ions into the electron cloud in front of the emitter electrode generally consists of filling the interelectrode gap with an ionized gas. Thermionic converters with gas in the interelectrode gap are designed to operate with such ionized species, typically utilizing cesium vapor. Utilization of a cesium vapor results in a space charge neutralization, effectively eliminating the detrimental deterrence of electron flow. Cesium also plays a dual role by decreasing the work function of the device, i.e. the rate of electrons leaving a surface, by absorbing onto the emitter and collector surfaces, thereby allowing greater electron emission. However, too great of a pressure of cesium in the interelectrode gap will cause excess collisions between cesium atoms and electrons leaving the emitter electrode, reducing the efficiency of conversion. Therefore, a careful, complex balance must be maintained in a cesium vapor system. The current apparatus bypasses the complexities and efficiency losses of such a system (and its related expense) by lowering the space-charge effect through reduction of spacing between electrodes to the order of microns (i.e., 1-10 microns).
Reducing the spacing between electrodes to the order of microns has proved impractical with conventional manufacturing techniques. Fitzpatrick (U.S. Pat. No. 4,667,126) teaches xe2x80x9cmaintenance of such small spacing with high temperatures and heat fluxes is a difficult if not impossible technical challenge.xe2x80x9d The present invention overcomes the difficulty of reducing spacing by microengineering. U.S. Pat. No. 6,294,858 to King, et al., xe2x80x9cMicrominiature Thermionic Convertersxe2x80x9d, which is hereby incorporated herein by reference, discloses a microminiature thermionic converter having a 1 micron electrode gap manufactured by integrated circuit (IC) semiconductor techniques. U.S. Pat. No. 6,299,083 to Edelson, xe2x80x9cThermionic Converterxe2x80x9d, also discloses a microminiature thermionic converter fabricated using MEMS techniques. Both King""s device and Edelson""s device are powered by an external source of heat; not by an internal, self-contained power source, as in the present invention.
Earlier thermionic converters relied on external heat sources (nuclear power, geothermal energy, solar energy, fossil fuel combustion, wood or waste combustion), which may not be readily available to a user especially if electricity is desired in powering a mobile miniature device.
The present invention, in contrast, with its incorporated thermal source, overcomes the very modern problem of mobility and also provides more choices for operating devices that do not necessarily need to be mobile. For example, devices that are fixed, but may need to be used in a limited space may not be able to harness the thermal energy sources used by earlier devices.
The apparatus of the present invention is a self-powered microthermionic converter. A preferred embodiment of the converter comprises an emitter electrode and a collector electrode, separated by a micron-scale spaced interelectrode gap, a self-contained (i.e., incorporated, integral) thermal power source in good thermal contact with the emitter electrode, and an electrical circuit connecting the collector electrode and emitter electrode through an external electrical load.
The interelectrode gap of a preferred embodiment is preferably less than about 10 xcexcm, more preferably, between approximately 1 xcexcm and 10 xcexcm, and most preferably, between approximately 1 xcexcm and 3 xcexcm. The interelectrode gap preferably comprises a vacuum. Alternate embodiments utilize cesium (or barium) vapor at a low vapor pressure, unlike the more common high vapor pressure cesium systems utilized in prior art inventions. The proposed alternate configuration, using low pressure cesium, differs from a Knudsen diode in that a small quantity of cesium is sealed into the present device during manufacture, whereas the Knudsen diode requires an external source of cesium (i.e., a cesium source apparatus).
A radioactive isotope can be used as the xe2x80x9cself-poweredxe2x80x9d thermal power source, such as alpha-emitting Curium-242, Curium-244, or Polonium-210. Alpha particles emitted from the isotope deposit their energy (heat) within the body of the isotope if the range of the alphas is much smaller than the physical dimensions of the body (e.g., the range of a 6 MeV alpha particle is about 13 microns in copper). If the body of the isotope is very well thermally insulated, then the deposited heat can raise the temperature to very high values, greater than 600 C.
The collector electrode and emitter electrode of the converter are preferably formed by depositing or growing at least one layer of thermionic electron emissive material on a substrate. The thermionic electron emissive material is preferably an alkaline earth oxide in combination with a refractory metal. Thermionic emissive materials can be selected from barium oxide, calcium oxide and strontium oxide; combinations of these oxides; along with additions of aluminum and scandium oxides, as adjunct oxides. The preferable refractory metal to incorporate into the electron emissive oxide is tungsten, but could also include rhenium, osmium, ruthenium, tantalum, and iridium, or any combination of these metals. Tungsten, rhenium, osmium, ruthenium and iridium, or any combination of these metals can also be used as terminating (capping) top layers on the oxide or mixed oxide/metal layer. Alternately, low-pressure alkali or alkaline earth metals, such as cesium and barium, can be used with a high work function metal like tungsten, tantalum, rhenium, osmium, ruthenium, molybdenum, iridium and platinum, or any combination of these metals. The oxides of like tungsten, tantalum, rhenium, osmium, ruthenium, molybdenum, iridium and platinum, or any combination of these metals can also be used with low-pressure alkali or alkaline earth metals, such as cesium and barium.
The emitter electrode length is preferably less than approximately 200 xcexcm, more preferably, between approximately 50 xcexcm and approximately 200 xcexcm, and most preferably, between approximately 50 xcexcm and approximately 100 xcexcm.
An electrical insulator may be disposed between non-interacting portions of the emitter electrode and collector electrode. A thermal heat barrier must be included to prevent heat loss from the source. The thermal heat barrier can be selected from alumina, quartz, aerogel, a multifoil system or a microheat barrier system. In the microheat barrier approach, multiple, highly-reflective surfaces are separated by micro-spikes or micro-posts and are fabricated using microfabrication techniques.
The preferred temperature for operation for the present invention is between approximately 850 K and approximately 1200 K. More preferably, the temperature is between approximately 1100 K and approximately 1200 K.
The present invention is also directed to a self-powered microthermionic converter with a diode having a collector electrode and an emitter electrode, a fuel cup, a thermal power source within the fuel cup and an interelectrode gap spaced between the emitter electrode and an edge region outside of the fuel cup. The outer surface of the fuel cup is coated with a thermionic electron emissive layer to form the emitter electrode. The edge region is coated with a thermionic electron emissive layer to create a collector electrode. The diode of the embodiment is in electrical contact with an electric circuit.
The present invention also includes methods for thermionic power conversion by placing an incorporated thermal power source in thermal contact with an emitter electrode. The heated emitter electrode emits electrons which travel across a micron spaced interelectrode gap to a collector electrode. Upon reaching the collector electrode, the electrons flow through an external resistive load that may be integral to the same micro-chip housing the self-powered thermionic device, or that may be external to the self-powered thermionic device. After traveling through this load, the electrons return to the emitter electrode, thereby completing an electrical circuit. The method can include utilization of an incorporated thermal power source where the source is enclosed within the emitter electrode.
The present invention further includes a method for manufacturing the self-powered microthermionic converter apparatus. A thermally and electrically insulating material is used as a substrate and forms a fuel cup with a thermal power source from the substrate through micromachining techniques. At least one thermionic electron emissive layer is deposited on the outer surface of the fuel cup to comprise an emitter electrode. A collector electrode is formed within the substrate outside of the emitter electrode by depositing at least one layer of a thermionic electron emissive material on at least one wall of the substrate. The thermionic electron emissive layer or layers are preferably formed through chemical vapor deposition techniques (CVD). CVD is preferred for non-planar geometries, however, RF sputter deposition, physical vapor deposition, reactive deposition, laser ablation, or electrophoretic deposition can be used, as well. A micron spaced interelectrode gap is located between the collector electrode and emitter electrode. Micromachining techniques are preferably used to form the fuel cup and substrate wall utilized as the collector electrode surface. The converter is preferably incorporated into a micromachined wafer.
The method of the present invention also comprises forming a fuel cup by forming a fuel grid, aligning the grid with the cups, inserting the sources in the cups, capping the cups, and dissolving the grid. The cap is preferably made of a highly reflective surface, non-reactive metal, such as gold.
A primary object of the present invention is to provide a mobile, miniature, self-powered thermionic converter.
Another primary object of the invention is to provide a thermionic power source of reduced size for incorporation into the converter.
Another object of the invention is to increase the efficiency of a thermionic converter by reduction in size of the interelectrode gap to micron-scale.
A primary advantage of the present invention is the small size of the invention due to the incorporation of a radioisotopic thermal heat source, which need only be utilized in minute amounts, and has a relatively long lifetime (e.g., months). The incorporation of the source removes the need for the external heat sources necessary with prior art devices. This both increases the mobility and decreases the necessary size of the converter in combination with the heat source.
Another distinct advantage of the current invention is the incorporation of the thermionic converter directly into the chip or other device it is intended to power. Such chips or devices can include MEMS, IMEMS, and micro fuel cell devices.
Other objects, advantages and novel features, and further scope of applicability of the present invention will be set forth in part in the detailed description to follow, taken in conjunction with the accompanying drawings, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.